1. Technical Field
This disclosure relates generally to an inkjet-type image forming apparatus, in which a recording head that ejects ink droplets onto a recording medium is moved in a main scanning direction perpendicular to a direction of conveyance of the recording medium to form an image on the recording medium while the recording medium is intermittently conveyed.
2. Description of the Background Art
In an inkjet-type image forming apparatus such as an inkjet printer, a recording head scans in a main scanning direction and ejects ink droplets onto a recording medium such as a sheet of paper to form an image on the sheet while the sheet is intermittently conveyed in a sub-scanning direction perpendicular to the main scanning direction.
Accuracy in conveyance of the sheet affects image quality in the inkjet-type image forming apparatus. Therefore, a pair of discharge rollers that discharges the sheet from the image forming apparatus is required to exert just enough conveyance force to reliably convey and discharge the sheet without affecting conveyance accuracy.
In addition, it takes time to dry the ink ejected onto the sheet. Therefore, a discharge roller serving as a drive roller that constitutes one of the pair of discharge rollers is generally disposed on a side opposite the side of the sheet on which the image is formed (hereinafter the “image formation side” of the sheet). By contrast, a spur that contacts the discharge roller and is driven by the discharge roller is generally provided on the same side as the image formation side of the sheet. Consequently, contact pressure of the spur against the discharge roller is minimized in order to prevent deterioration in image quality.
In a case in which a reversal unit that reverses the sheet during duplex image formation is provided downstream from an imaging range of the recording head in a direction of conveyance of the sheet, the image forming apparatus is generally further provided with a curved conveyance path through which the sheet is conveyed. The direction of conveyance of the sheet is changed through the curved conveyance path, such that the sheet having an image on a front side thereof is either discharged from the image forming apparatus or is reversed to form an image on a back side thereof. When the sheet passes through the curved conveyance path, conveyance resistance of the sheet is increased. Therefore, it is necessary to increase the conveyance force of the pair of discharge rollers, by increasing the contact pressure of the spur against the discharge roller. Although an increase in the contact pressure of the spur against the discharge roller increases the conveyance force of the pair of discharge rollers, it also degrades image quality. In addition, an increase in the conveyance force adversely affects accuracy in conveyance of the sheet.
Some image forming apparatuses have two pairs of discharge rollers to increase the total conveyance force of the pairs of discharge rollers without changing a conveyance force per pair of discharge rollers. For the purpose of stabilizing a trailing edge of the sheet after the sheet passes through a pressing roller, the first pair of discharge rollers is provided downstream from a recording unit in a direction of conveyance of a sheet and the second pair of discharge rollers is provided downstream from the first pair of discharge rollers. The first pair of discharge rollers biases the trailing edge of the sheet in a direction to separate the sheet from the recording unit so as to discharge the sheet from the image forming apparatus.
In another approach, the image is formed on the sheet while the sheet is conveyed horizontally. The axial center of an upper roller constituting one of the second pair of discharge rollers is displaced in the direction of conveyance of the sheet from the axial center of a lower roller constituting the other one of the second pair of discharge rollers so as to prevent deterioration in image quality.
However, in the above-described configurations in which the two pairs of discharge rollers are provided so as to increase the total conveyance force of the pairs of discharge rollers, the sheet is pulled by the conveyance force of the second pair of discharge rollers when a leading edge of the sheet is sandwiched by the second pair of discharge rollers either while or before the last line of the image is being formed on the sheet by the recording unit, thus adversely affecting sheet conveyance accuracy.